This invention relate to a control system for a direct injection engine of spark ignition type which is provided with a fuel injection valve for directly spraying fuel into combustion chambers and a catalyst in an exhaust passage.
As disclosed, for example, in Japanese Unexamined Patent Publication No. 11-36959, a conventional control system is provided with a fuel injection valve for directly spraying fuel into combustion chambers and a controller for causing the fuel to be sprayed during a compression stroke to carry out a stratified combustion in a specific low engine speed/low load operating region while causing it to be sprayed during an intake stroke to carry out a premix combustion (uniform combustion) in other operating regions.
In the engine of this type, among the operating regions where uniform combustion is carried out, a high engine speed/high engine load region is referred to as an enriched region, and a region having a lower engine speed and a lower engine load than the enriched region is referred to as a stoichiometric air-fuel ratio region. In the stoichiometric air-fuel ratio region, a required output is achieved by controlling a fuel injection amount and an intake air amount such that an air-fuel ratio becomes a stoichiometric air-fuel ratio, and satisfactory emissions are maintained by improving an exhaust gas purifying performance by a catalyst in the exhaust passage. On the other hand, in the enriched region, an output is increased by increasing the fuel injection amount to decrease the air-fuel ratio below the stoichiometric air-fuel ratio, and a rise in exhaust gas temperature is suppressed by a thermal capacity and latent heat by vaporization of an excessive fuel to thereby prevent an excessive heating of the catalyst provided in the exhaust passage to ensure a satisfactory reliability.
In the conventional system, it is desirable to make the enriched region as small as possible for the improvement of fuel consumption at high speeds (fuel consumption in a high engine speed region) since the fuel is excessively fed in the enriched region to increase an amount of fuel consumed. However, extension of the stoichiometric air-fuel ratio region to the high engine speed/high load region by reducing the enriched region is not preferable in terms of reliability since a larger amount of heat is generated at the stoichiometric air-fuel ratio to thereby make the exhaust gas temperature likely to rise.
It is an object of the present invention to provide a control system for a direct injection engine of spark ignition type which is free from the problems residing in the prior art.
According to an aspect of the invention, a control system is adapted for a direct injection engine of sparking ignition type including a catalyst in an exhaust passage and a fuel injection valve for directly spraying fuel into a combustion chamber. The control system is provided with a setter for setting an enriched region where an air-fuel ratio is smaller than a stoichiometric air-fuel ratio in a high engine speed and load area of an operating region of the engine, setting a stoichiometric air-fuel ratio region where the air-fuel ratio is equal to the stoichiometric air-fuel ratio in an area of the operating region of the engine having lower engine speed or lower engine load than the enriched region, and setting a lean region where the air-fuel ratio is larger than the stoichiometric air-fuel ratio between the stoichiometric air-fuel ratio region and the enriched region; and an air-fuel ratio controller for controlling the air-fuel ratio based on the setting by the setter.
In the direct injection engine of sparking ignition type, the lean region where the air-fuel ratio is larger than the stoichiometric air-fuel ratio is set between the enriched region having high engine speed and load and the stoichiometric air-fuel ratio region having lower engine speed and load than the enriched region. Accordingly, rise in exhaust gas temperature is suppressed in the lean region, thereby preventing superheating of the catalyst and enhancing combustion efficiency to improve fuel consumption.
These and other objects, features and advantages of the present invention will become more apparent upon a reading of the following detailed description and accompanying drawings.